Low-cost hypereutectoid bearing steel containing molybdenum and chromium

ABSTRACT

A LOW-COST HYPEREUTECTOID BEARING STEEL HAVING CHARACTERISTICS EQUAL TO OR SUPERIOR TO HIGHER PRICED STEELS OF THIS TYPE AND CONTAINING AS ESSENTIAL ALLOYING ADDITIONS .801.10% CARBON, .50-1.00% MANGANESE, .20-60% CHROMIUM AND .05-25% MOLYBDENUM. A STEEL OF THIS TYPE, USUALLY IN THE FORM OF WIRE, ROD, BAR, FORGINGS, RINGS OR TUBING, IS ANNEALED AT A MAXIMUM TEMPERATURE OF 1400* F. FOR THIRTEEN HOURS TO PRODUCE AN ANNEALED MICROSTRUCTURE COMPRISING SPHEROIDIZED CARBIDES IN A MATRIX OF FERRITE. AFTER FORMING INTO BEARING RACES OR BALLS, THE FORMED PRODUCT IS QUENCHED FROM 1550* F. AND THEREAFTER TEMPERED TO PRODUCE A STEEL HAVING A ROCKWELL C HARDNESS OF ABOUT 60 OR GREATER AND EXCELLENT FATIGUE STRENGTH.

March 26, 1974 Y c. P. wr-:lGEL 3,799,766

LOW-COST HYPEREUTECTOID BEARING STEEL CONTAINING MOLYBDENUM AND CHROMIUM Original Filed May 26, 1971 Y s a a: se QE w) United States Patent O 3,799,766 LOW-COST HYPEREUTECTOID BEARING STEEL CONTAINING MOLYBDENUM AND CHROMIUM Charles P. Weigel, Canton, Ohio, assignor to The Timken Company, Canton, Ohio Original application May 26, 1971, Ser. No. 146,911, now

Patent No. 3,704,183, dated Nov. 28, 1972. Divided and this application July 10, 1972, Ser. No. 270,492

Int. Cl. C22c 39/14 U.S. Cl. 75-126 C 3 Claims ABSTRACT OF THE DISCLOSURE A low-cost hypereutectoid bearing steel having characteristics equal to or superior to higher priced steels of'this type and containing as essential alloying additions .80- 1.10% carbon, .S-1.00% manganese, .20-.60% chromium and .0S-.25% molybdenum. A steel of this type, usually in the form of wire, rod, bar, forgings, rings or tubing, is annealed at a maximum temperature of l400 F. for thirteen hours to produce an annealed microstructure comprising spheroidized carbides in a matrix of ferrite. After forming into bearing races or balls, the formed product is quenched from 1550 F. and thereafter tempered to produce a steel having 'a Rockwell C hardness of about 60 or greater and excellent fatigue strength.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a division of copending application Ser. No. 146,911, led May 26, 1971, now Pat. No. 3,704,183.

BACKGROUND OF THE INVENTION As is known, AISI 52100 steel has been used extensively, and almost exclusively, for ball bearing applications. Such a steel contains as essential alloying additions, .9S-1.10% carbon, .2S-.45% manganese and 13G-1.60% chromium. The relatively high amount of chromium is added to assist in hardenability; however this materially increases the cost of the product.

In the manufacture of bearings from AISI 52100 steel, tubular shapes, bars, forgings and rings (for forming bearing races) and wire or rod (for forming balls) are initially annealed at about l450 F., maximum temperature, and furnace cooled at a controlled rate for twentytwo hours, followed by air cooling. In the heat treating process, spheroidized carbides are formed in a matrix of ferrite to produce a product having good machinability, cold heading and/or cold forming properties. After the machining, cold heading or forming operation, the balls and races are quenched in oil from about 1550 F. and then tempered to achieve a Rockwell C hardness of 60 or greater. Of course, the steel must also have good fatigue strength.

While various alloys with about .6 or .7% carbon have been proposed in the past as low-cost substitutes for AISI 52100 bearing steel, these generally result in greater difculty in obtaining spheroidized annealed microstructures or fatigue strengths inferior to those of AISI 52100.

3,799,7 66 Patented Mar. 26, 1974 ICC SUMMARY oF THE INVENTION The present invention resides in the discovery that a high carbon bearing steel can be prepared which contains substantially less total alloy content than conventional bearing steels, without sacrificing the hardenability, fatigue strength and processing characteristics of higher alloy bearing steels. At the same time, the annealing time required to achieve adequate carbon diffusing to produce a spheroidized microstructure is materially reduced with the steel of the invention.

Specifically, it has been found that the carbon content can be reduced somewhat in a high carbon steel, and that the chromium content can be reduced to as little as .20%, by increasing the manganese content and by adding suitable amounts of molybdenum. The steel of the invention contains as essential constituents, in addition to iron and carbon, manganese, molybdenum and chromium.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single figure drawing comprising a plot showing the superior fatigue strength of the steel of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The bearing steel of the invention has the following broad and preferred compositions:

ANALYSIS-PERCENT BY WEIGHT Nickel and silicon are relatively unimportant in achieving the desired characteristics of the alloy and can be classified as residual amounts. .20-.35% silicon is that amount found in most alloy steels. Similarly, a certain amount of sulfur is found in every steel; however, it is desired to deliberately include .008-.04% sulfur to improve machinability and fatigue strength. As was mentioned above, carbon, manganese, chromium and molybdenum are the essential additions, the chromium and molybdenum acting to increase hardenability. In the preferred analysis given above, percent by weight of carbon is however it should be understood that this can vary plus or minus ve points, in accordance with usual metallurgical practices.

The steel of the foregoing composition is melted, formed into tubing, forgings, rings, bars, rod, wire or other shapes, and then annealed at a maximum temperature of 1400 F. for thirteen hours in order to achieve a spheroidized microstructure comprising spheroidized carbides in a matrix of ferrite. In the annealing step, the steel is passed through eleven zones of a roller hearth furnace at a constant speed, the time in each zone being about one-eleventh of the total time of thirteen hours. 'Ihe temperatures of the respective zones are shown in the following Table I:

TABLE I Annealing temperatures in eleven zone roller hearth furnace Thus, the product is heated to 1250 F., 1300 F. and 1350 F. ea-ch for about one hour and eleven minutes, followed by heating to 1400 F. for about two hours and twenty-two minutes, and finally cooled to 1300 F. for the remainder of the total of thirteen hours, followed by an air cool. The annealing time of thirteen hours at a maximum temperature of 1400 F. compares favorably with the required annealing time of twenty-two hours at a maximum temperature of 1450 F. for AISI 52100 steel. Furthermore, the shorter annealing time reduces the cost of the product and results in less scaling and less decarburization.

The annealed product is then cold headed or cold formed into balls and machined or formed into races which are thereafter heated to a temperature of l550 F., followed by quenching in oil. In this condition, the product has a Rockwell C hardness of about 63-66. The nal step in the process is to temper the product at 300 F. for about 1 to 2 hours to achieve a Rockwell C hardness of 60 or greater.

The resulting product has a density of 0.283 pound per cubic inch, a specic gravity of 7.83, a modulus of elasticity of 29 106 pounds per square inch and a modulus of rigidity of 12 106 pounds per square inch. In addition, the product has excellent fatigue strength, comparable to that of the higher alloy AISI 52100 bearing steel and much better than AISI 5160 which is sometimes used as a low-cost substitute for bearing steels. This is shown in the accompanying drawing wherein curve is that for the steel of the invention; whereas curve 12 is that obtained from fatigue tests conducted on AISI 5160 steel. The steel of the invention has an L10 fatigue life (i.e., a probability of 10% failures, or 90% survival) after about 10 million stress cycles; whereas AISI 5160 steel has an L10 fatigue life after only 6.5 million stress cycles. Note that the improvement in fatigue life persists at 50% survival. Thus, the steel of the invention has an L50 fatigue life of 45 million stress cycles `as oompared with 25 million stress cycles for AISI 5160.

Typical hardenability of the steel of the invention, determined with the Jominy hardenability test, is shown in the following Table II:

TABLE II Typical hardenability (normalize 1650 F., anneal, quench 1550 F.)

Rockwell C hardness at corresponding point J Distance from quenched end in V16 inch:

Minimum hardness on 0.5 inch section heated to 1525 ,F.ilO" for 2O minutes and oil quenched-63.0 Rockwell C. This compares favorably with the more expensive, conventional bearing steels.

The effect of the austenitizing temperature on the steel of the invention is shown in Table III:

TABLE III [Heat treated properties-elect of austenitizing temperature] Austenitizing As-quenehed Hardness temperature hardness after 300 F.) (Rockwell C) temper Note that the austenitizing temperature has very little eiect on the as-quenched hardness. Finally, the effect of the tempering temperature on the steel of the invention is shown in Table IV:

TABLE IV [Effect of tempering temperature (1,550D F. quench)] Hardness Rockwell C NoTE.-As-quenched hardness:65.0 Rockwell C. Note that in order to achieve the desired Rockwell C hardness of 60, tempering temperatures up to 400 F. may he employed. The tempering time is not particularly critical.

Although the invention has been shown in connection with certain specific examples, it will be readily apparent to those skilled in the art that various changes in composition and method steps can be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. An alloy steel consisting essentially of .80-1.10% carbon, .S0-1.00% manganese, .20-.60% chromium, .05- .25% molybdenum and the balance substantially all iron.

2. The alloy steel of claim 1 wherein carbon is present in the amount of .95% by weight, manganese is present in the amount of .5 0-.80% by Weight, chromium is present in the amount of .4D-.60% by weight and molybdenum is present in the amount of .0S-.15% by weight.

3. The alloy steel of claim 1 containing .008.04% by weight sulfur.

References Cited UNITED STATES PATENTS 2,763,545 9/1956 Breeler 75-126 C 2,395,687 2/1946 Selmi 75-126 C 3,250,610 5/1966 Huber 75-126 C 3,306,734 2/1967 .Aksoy 75-126 C 3,060,016 10/1962 Melloy 73-126 C 3,251,682 5/1966 Wada 75-126 C HYLAND BIZOT, Primary Examiner 

